Metal halide lamps are known in the art and are described in, for instance, EP 0215524 and WO 2006/046175. Such lamps operate at high pressures and have burners or ceramic discharge vessels comprising ionizable gas fillings of, for instance, NaI (sodium iodide), TlI (thallium iodide), CaI2 (calcium iodide) and/or REIn. REIn refers to rare-earth iodides. Characteristic rare-earth iodides for metal halide lamps are CeI3, PrI3, NdI3, DyI3, and LuI3.
Most present-day discharge vessels for metal halide lamps have a spherical shape, as described in, for instance, DE 20205707, a cylindrical shape, as described in, for instance, EP 0215524 or WO 2006/046175, or an extended spherical shape as described in, for instance, EP 0841687 (U.S. Pat. No. 5,936,351). The latter document describes a ceramic discharge vessel for a high-pressure discharge lamp constituted by a cylindrical central part and two hemispherical end pieces, wherein the length of the central part is smaller than or equal to the radius of the end pieces. In this way, the isothermy of the discharge vessel is improved.